lntegrated management strategy for improving the grain yield and nitrogen-use efficiency of winter wheat

XU Hai-cheng, DAl Xing-long, CHU Jin-peng WANG Yue-chao, YlN Li-jun MA Xin DONG Shuxin HE Ming-rong

1 State Key Laboratory of Crop Biology/Key Laboratory of Crop Ecophysiology and Farming System, Ministry of Agriculture/Agronomy College, Shandong Agricultural University, Tai’an 271018, P.R.China

2 Administrative Committee of Yellow River Delta Agri-High-Tech Industry Demonstration Zone, Dongying 257347, P.R.China

3 College of Plant Science and Technology, Huazhong Agricultual University, Wuhan 430070, P.R.China

1. lntroduction

Over the decades, the exponential increase in population and gradual decrease in arable land has imposed tremendous pressure on researchers to increase wheat(Triticum aestivumL.) productivity. High application rates of chemical fertilizers, especially nitrogen (N) fertilizers,are commonly used to increase wheat yield, and China is the largest consumer of N fertilizers globally. In 2010,32.6 million tons of N fertilizers, approximately 31.3% of the global N consumption, were applied in China, and approximately 13.8% of this N consumption was used for wheat production (Heffer 2013). However, these high N inputs have not enhanced grain yield, but instead have had negative environmental impacts through nitrate leaching and nitrous oxide emissions (Sylvester-Bradley and Kindred 2009; Hawkesford 2014). Consequently, agricultural N use efficiency (NUE) in China is extremely low (Zhanget al.2008). Therefore, over-application of fertilizers could be one of the main reasons that China’s grain production has not increased proportionally with the increase in chemical fertilizer consumption in the last decades (Jiaoet al. 2016).

The wheat production area in China is divided into a spring wheat production zone, a winter wheat production zone, and a spring-winter wheat mixed production zone.The spring wheat production zone includes the northeastern,northern, and northwestern regions. The winter wheat production zone includes the northern, Huang-Huai,the middle and lower reaches of the Changjiang River,southwestern, and southern China regions. The Xinjiang and Qinghai-Tibet regions are included in the spring-winter wheat mixed production zone. The Huang-Huai region is the most important wheat production zone, and accounts for approximately 45 and 48% of the national planting area and output, respectively. There is a pressing demand for synergetic improvement of the grain yield and NUE of wheat in the Huang-Huai winter wheat production zone.

The demand for crop production security and the demands of resource and environmental management require crops with both high yields and high N efficiency(Penget al. 2006). Breeding of N efficient wheat cultivars is one method to reduce the application rate of N fertilizers while maintaining acceptable yields (Foulkeset al. 2009).However, wheat yield and N efficiency have been shown to be directly affected by agronomic practices, such as seeding rate (Berryet al. 2000; Daiet al. 2013), N management(Fageria and Baligar 2005; Shiet al. 2012), and sowing date (Sunet al. 2007; Gaoet al. 2012).

NUE was defined as the grain dry matter yield per unit of available N (from both soil and fertilizer) and can be divided into two components: N uptake efficiency (UPE) and N utilization efficiency (UTE; Mollet al. 1982). UPE depends mainly on the aboveground N uptake (AGN) and available N. UTE can be calculated by dividing the N harvest index(NHI) by the grain N concentration (GNC; Zhanget al. 2016).Thus, agronomic management can be used to improve NUE by recovering more N from both soil and fertilizer (better UPE) and/or utilizing the absorbed N to produce more grain(better UTE).

Appropriately increasing the seeding rate is considered the basis for achieving greater grain yield (Hiltbrunneret al.2007) and N accumulation (Arduiniet al. 2006; Daiet al.2014) in winter wheat. Daiet al. (2013) concluded that proper management of the seeding rate can lead to both higher grain yield and NUE through increasing the UPE and AGN. A positive quadratic relationship was observed between wheat grain yield and N application rate(Hawkesford 2014), whereas NUE, UPE, UTE, and NHI were found to decrease with an increased N application rate (Abrilet al. 2007; Pasket al. 2012; Zhanget al. 2016).Both high seeding rates and high N levels have also been shown to increase the risk of lodging because stems tend to be weaker under high-input conditions (Berryet al.2000; Loyceet al. 2008). In addition, Zhanget al. (2015)have suggested that decreasing the N application rate and increasing the seeding rate allows efficient absorption of N at deep soil depths and could lead to high grain yield, NUE,UPE, and UTE in winter wheat. An improper N application date (during the early vegetative stage) was found to frequently result in the poor use of N fertilizer because of the low N uptake due to the small amounts of roots in winter wheat (Fageria and Baligar 2005). Extensive experiments have demonstrated that adjusting the N application date and rate to synchronize N supply and crop demand can result in high grain yield and N efficiency (Cassmanet al. 2002;Shanahanet al. 2008; Luet al. 2015). Moreover, applying a appropriate top dressing ratio can lead to increased grain yield and plant N uptake (Shiet al. 2012). For example,reducing the N application rate while increasing the top dressing ratio achieved synchronous enhancement of both grain yield and N agronomic efficiency, as well as apparent N recovery efficiency (López-Bellidoet al. 2005). In general,delayed sowing of wheat markedly increases stem strength by increasing both stem diameter and wall thickness, thereby significantly improving lodging resistance (Berryet al. 2000);however, delayed sowing shortens the vegetative stage (Sunet al. 2007) and thus reduces the spikes (Thillet al. 1978)and ultimately decreases grain yield (Ortiz-Monasterioet al.1994), AGN and UPE (Daiet al. 2017). Hence, the seeding rate should be increased to offset the detrimental effects of delayed sowing date through increasing main stem spikes per unit area (Shahet al. 1994). Unchanged (McLeodet al.1992), increased (Ozturket al. 2006; Alignanet al. 2009),and decreased (Suarez-Tapiaet al. 2017) GNC values have been observed with delaying sowing time. Properly sowing late could maintain grain yield and NUE through increasing UTE as a result of a reduced GNC (Daiet al. 2017).

Few studies focusing on the effects of combinations of agronomic options (such as seeding rate, sowing date,and nutrient management) on grain yield and NUE have been reported for winter wheat. It is possible that the integrated effects of management strategies are not the simple result of stacking each factor. Strengthening the beneficial effects and mitigating the detrimental effects are the primary goals of wheat cultivation studies. Because integrated management could have differential impacts on yield formation and N use in winter wheat, we performed field experiments that included four treatments and evaluated the integrated effects of these treatments on grain yield and yield components and quantified N uptake and utilization in the Huang-Huai winter wheat production zone. The objective of this study was to determine an optimal management strategy to achieve high grain yield, high NUE, and high economic benefit. We also investigated the key components of NUE including AGN, UPE, UTE, NHI, and GNC to account for differences in NUE.

2. Materials and methods

2.1. Site and growing conditions

Field experiments were conducted as a part of a long-term experiment from 2008 located in Dongwu Village, Dawenkou Town, Tai’an City, Shandong Province, China. The present study was conducted from 2013 to 2015. The area has a semi-humid continental temperate monsoon climate. The annual solar duration was at least 2 627 h, the annual cumulative temperature above 10°C was at least 4 213°C and the annual frost-free period was 195 days. The rainfall totals during the entire growing season in 2013 (157.20 mm),2014 (139.50 mm), and 2015 (164.30 mm) were all below the 40-year average (188.31 mm). The rainfall and mean temperature during the experimental period are shown in Fig. 1.

The rotation of winter wheat-summer maize was performed based on the double-cropping system used in this region. The straw of the previous summer maize crop was returned every year. The soil was sandy loam, and the nutrient status before sowing in each of the three seasons is shown in Table 1.

Fig. 1 Rainfall and mean temperature recorded during the 2013 to 2015 wheat growing seasons in Dongwu Village, Dawenkou Town, Tai’an City, Shandong Province, China.

2.2. Experimental design and treatment

We evaluated the effect of four integrated management strategies on grain yield and NUE: current practice(T1), improvement of current practice (T2), high-yield management (T3), and integrated soil and crop system management (T4). For T1, the traditional practices of local farmers were followed; for T2, the farmers’ practices were modified to improve yield and NUE; for T3, the aim was to maximize grain yield by optimizing soil and management practices regardless of the cost of resource inputs; for T4,the aim was to test an optimal combination of seeding rate,sowing date, date and rate of fertilizer application, and topdressing ratio. Plants were arranged in a randomized block design with four replications for each treatment. The plot size was 3 m×40 m with 12 rows (0.25 m between rows).The winter wheat cultivar Tainong 18, which is popular in this region, was chosen for the experiment.

The seeding rate, sowing date, nutrients, and irrigation management used for the four treatments are provided in Table 2. Seeding rates increased by 75 seeds m–2with each treatment from T1 (225 seeds m–2) to T4 (450 seeds m–2). The sowing dates were delayed from T1 (5th Oct.) to T2 and T3 (8th Oct.), and to T4 treatment (12th Oct.). For each treatment 210–315 kg N ha–1(as urea), 90–210 kg P2O5ha–1(as calcium superphosphate), and 30–150 kg K2O ha–1(as potassium chloride) were applied. The ratio of basal application to topdressing (regreening stage) of N fertilizer for T1 was 6:4. The ratios for T2, T3, and T4 were 5:5, 4:6, and 4:6, respectively, whereas the N topdressing was applied at the jointing stage. The P fertilizers in all four treatments were applied as a basal fertilizer. The K fertilizer for T1 and T2 was applied as a basal fertilizer prior to planting, while the ratios of basal application to topdressing(jointing stage) of K fertilizer for both T3 and T4 were 6:4. T1 was generally irrigated five times, T2 and T3 were irrigated four times, and T4 was irrigated three times. The irrigation amount was 70 mm each time and was performed with a mobile plastic hose connected to a tap and the irrigation amount was controlled by a water meter installed at the discharge end of the hose. No significant incidences ofpests, diseases, or weeds were observed for any treatment.Harvests were performed on 13th June 2013, 8th June 2014,and 10th June 2015.

Table 1 The organic matter, total nitrogen (N), available phosphorous (P), and available potassium (K) in soil sampled at a depth of 0–20 cm before seeding in 2013–2015

Table 2 The seeding rate, sowing date, nutrients, and irrigation management used for the four experimental treatments

2.3. Crop measurements

Representative soil samples (0–100 cm) were taken prior to the application of fertilizers and sowing of winter wheat.Fresh soil samples were extracted with 1 mol L–1KCl to determine the concentrations of mineralized N (NH4+-N and NO3

–-N) in the soil using a continuous flow analyzer(Bran+Lubbe, Norderstedt, Germany). The available N was the sum of N from fertilizer and the mineralized N in 0–100 cm depth soil.

At anthesis and maturity stages, plants from a 1.0-m2area near the middle of each plot were sampled and separated into stems, sheaths, lamina, and spikes.The dry matter weight of each plant component was determined after drying at 70°C to a constant weight and then weighing. The spikes at maturity stage were divided into grains and chaff, after which they were ovendried and weighed. N concentration was determined using the Kjeldahl method. The N accumulation of each plant component was calculated by multiplying the N concentration and dry matter weight. AGN was defined as the sum of N accumulation of each plant component.

Tiller number and spike number per unit area were determined at jointing stage and before harvest stage by counting all shoots in a 2.0-m2quadrant, which contained four rows, in each plot. At harvest time, plants were collected from a 3.0-m2sample area (2.0 m×6 rows) in each plot to measure grain yield. Grain number per spike was measured by counting the grains of 30 randomly selected spikes in each plot before harvest.

A total of 30 randomly selected spikes in each plot were sampled at 3-day intervals from 3 to 36 days post-anthesis(DPA) in 2015. All grains in each spike were removed and then oven-dried and weighed. The grain filling process wasfitted by Richards’ (1959) growth equation, as described by Zhuet al. (1988):

The grain filling rate (GR) was calculated as the derivative of eq. (1):

Where, W is the grain weight (mg); A is the final grain weight (mg); t is DPA; and B, K, and N are coefficients determined by regression. The active grain filling period was defined as the period where W ranged from 5% (t1)to 95% (t2) of A. The average grain filling rate during this period (from t1 to t2) was calculated.

2.4. Calculation of N evaluating indicator

NUE (kg kg−1) was calculated as grain dry matter yield per unit N available (from both soil and fertilizer). NUE can be partitioned into UPE and UTE (Mollet al. 1982).

UPE (%) is defined as aboveground N uptake (AGN) in all the parts of the crop at maturity stage as a fraction of N available (from soil and fertilizer, Mollet al. 1982).

UTE (kg kg−1) is the grain dry matter yield divided by AGN.UTE can also be defined as the ratio of the N harvest index(NHI, %) to grain N concentration (GNC, %; Foulkeset al.2009; Barracloughet al. 2010).

NHI is defined as the proportion of AGN in grain at maturity stage (Foulkeset al. 2009).

2.5. Calculation of economic benefits

The costs of the four treatments were calculated based on the local unit price. The input included the fertilizer,mechanical operation, irrigation, labor, pesticide, and seeds.The output of each treatment was calculated based on the average grain yield across the three years and the price of wheat (2.25 CNY kg–1). The net profit was also calculated.

2.6. Statistical analysis

Our results were analyzed using DPS ver. 7.05 software(Hangzhou Ruifeng Information Technology Co., Ltd.,China). Multiple comparisons were performed after a preliminaryF-test. Differences between means were evaluated based on the least significant difference atP＜0.05(LSD0.05).

3. Results

3.1. Grain yield and yield components

Grain yield and yield components differed significantly between years and treatments, whereas the interaction effect was non-significant (Table 3). In all three years,grain yields among the treatments showed the following trend: T3 (high-yield management)＞T4 (integrated soil and crop system management)＞T2 (practice improved over current practice)＞T1 (current practice) (Fig. 2 and Table 3).T3 consistently achieved the highest grain yield, whereas T1 produced the lowest grain yield. Grain yield obtained with the T4 treatment averaged across all three years was 95.85% of that with T3 and was significantly higher than that with T1 and T2 by 21.72 and 6.10%, respectively. Grain yield obtained in 2014 and 2015, respectively, was higher than in 2013, whereas no significant differences in yield were observed between 2014 and 2015.

Table 3 Grain yield, spike number per unit area, grain number per spike, and thousand-kernel weight under different integrated treatments from 2013 to 2015

Fig. 2 Quadrant graph of grain yield and nitrogen use efficiency(NUE) under four integrated treatments across three growing seasons. Dotted lines represent the overall average grain yield and NUE of all integrated treatments (T1–T4, current practice,improvement of current practice, high-yield management, and integrated soil and crop system management, respectively)over three years. Quadrants (Q) are labeled 1–4 and exhibit the integrated treatments that are high or low yielding and high or low NUE.

The spike number per unit area among the treatments across the three years had the following rank order:T4＞T3＞T2≥T1. For grain number per spike, the order was T1＞T2≈T3＞T4. No significant difference in grain number per spike was observed between T2 and T3. Thousand-kernel weight among the treatments across three years followed the ranking: T2＞T3≈T4＞T1, where no significant difference was observed between T3 and T4 (Table 3).

3.2. Tiller number at jointing stage, ear-bearing tiller percentage, single-spike dry weight, and single-stem biological yield at anthesis stage, average grain filling rate, and active duration of grain filling

Over three years, tiller number at jointing stage ranged from 1 574.67 to 2 145.00 tillers m–2for all treatments(Table 4) and showed the following trend: T4≈T3＞T1≥T2.The ear-bearing tiller percentage exhibited the following trend: T2≈T4＞T3≈T1, where no remarkable differences were observed between T2 and T4 and between T1 and T3.

Over three years, the single-spike dry weight at anthesis stage among the treatments showed the following trend:T1＞T2≈T3＞T4. The single-stem biological yield at anthesis stage showed a similar trend except in 2013, where the ranking of treatments was: T1≈T3＞T2＞T4 (Table 4). Both single-spike dry weight and single-stem biological yield at anthesis stage were positively correlated with grain number per spike (Fig. 3-A and B).

The final grain weight was determined based on the grainfilling rate and active duration. The average grain filling rate among the treatments exhibited the following trend:T2＞T3≈T4＞T1. For the active duration of grain filling, the order was T1≈T3≈T4＞T2 (Table 4). Correlation analysis demonstrated that grain weight was positively related to the average grain filling rate but was not correlated with the active duration of grain filling (Fig. 3-C and D).

Table 4 Tiller number at jointing stage, ear-bearing tiller percentage, single-spike dry weight at anthesis stage, single-stem biological yield at anthesis stage, average grain filling rate, and active duration of grain filling under different integrated treatments from 2013 to 2015

Fig. 3 The relationshipbetween grain number per spike and single-spike dry weight at anthesis (A) and single-stem biological yield at anthesis stage (B), and the relationship between thousand-kernel weight and average grain filling rate (C) and active duration of grain filling (D). ＊＊, significant difference at the P＜0.01 level.

3.3. NUE and its components

NUE NUE was affected significantly by year, treatment and their interaction (Table 5). NUE ranged from 12.67 to 22.51 kg kg–1among the treatments and across the three years showed the following rank order: T2＞T4＞T3＞T1(Fig. 2 and Table 5). NUE averaged across three years for T4 was 25.62 and 51.91% higher than that for T3 and T1,respectively.

UPE, available N and AGNUPE was affected significantly by year, treatment and their interaction. UPE was the highest for T2 and the lowest for T1 over the three years.No significant differences in UPE were observed between T2 and T4 in 2014 and 2015. UPE averaged across three years for T4 was 16.62 and 50.75% higher than that for T3 and T1,respectively (Table 5). Correlation analysis demonstrated that NUE was positively related to UPE (Table 6).

The UPE was determined based on both available N and AGN. Available N differed markedly between all treatments with a ranking of T3≥T1＞T4＞T2 (Table 3). UPE was negatively related to available N and positively related to AGN (Table 4). We observed significant effects of year,treatment, and their interaction on AGN. The AGN at maturity stage for all treatments followed a ranking of T3＞T4＞T2＞T1,which was similar to the ranking observed for grain yield.AGN averaged across all three years for T4 was 88.92% of the average AGN for T3 and was significantly higher than the averages for T1 and T2 by 20.72 and 10.38%, respectively(Table 3). We observed a positive correlation between grain yield and AGN at maturity stage (Table 4).

UTE, NHl, and GNCThe effects of year, treatment, and their interaction on UTE were significant. UTE was the highest for T2 and the lowest for T3 over the three years. The UTE for T4 was significantly higher than that for T3, but was slightly lower than that for T2. However, we observed inconsistent trends between T1 and T4 over the three years. The UTE for T4 was higher than that for T1 in 2013 and 2015 but was the same in 2014. The UTE averaged across three years for T4 was 7.74% higher than that for T3 (Table 5).Correlation analysis demonstrated that NUE was positivelyrelated to UTE (Table 6).

Table 5 N use efficiency (NUE), N uptake efficiency (UPE), N utilization efficiency (UTE), available N, aboveground N uptake(AGN), N accumulation in grains (N-grain), N harvest index (NHI), and grain N concentration (GNC) under different integrated treatments from 2013 to 2015

Table 6 Correlation coefficients for grain yield, N use efficiency (NUE), and NUE components1)

N accumulation in grains per unit area was affected significantly by year, treatment, and their interaction. N accumulation in grains at maturity stage exhibited the same ranking among the treatments as AGN. N accumulation in grain per unit area averaged across all three years for T4 was 91.98% of that for T3 and was significantly higher than that for T1 and T2 by 26.34 and 9.70%, respectively.NHI was affected significantly by year, treatment, and their interaction. Over three years, the NHI was the largest for T2 and T4 and the smallest for T1. No significant differences in NHI were observed between T2 and T4 over the three years.The NHI for T4 averaged across three years was 4.65 and 3.46% higher than that for T1 and T3, respectively (Table 5).GNC varied significantly between years and treatments,whereas their interaction effect was non-significant. Over the three years, GNC was the highest for T3 and the lowest for T1, and no significant difference was observed between T1 and T2. GNC averaged across the three years for T4 was 4.04% lower than that for T3 (Table 5). Correlation analysis also demonstrated that UTE was positively related to NHI but negatively related to GNC (Table 6).

3.4. Economic benefits

The difference in the input costs between the treatments exhibited the following trend: T3＞T1＞T2≈T4. Based on the same unit price of wheat, the variation in output among the four management strategies followed the same rank order as grain yield. The net profit showed the following rank order: T4＞T2＞T3＞T1, and the net profit for T4 was 174.94,22.27, and 28.10% higher than that for T1, T2, and T3,respectively (Table 7).

4. Discussion

In actuality, wheat yield is determined by the specific balance of yield components. Agronomic practices can affect yield components by altering tillering development,the differentiation and degradation of young spikes, and the grain filling process (Gonzálezet al. 2011a; Luet al. 2014,2016; Ferriseet al. 2015).

Generally, low seeding rates increase the spike number per plant and the grain number per spike, but decrease the spike number per unit area, whereas the opposite occurs with high seeding rates (Tompkinset al. 1991; Whaleyet al. 2000; Ozturket al. 2006). The effect of seeding rate on thousand-kernel weight is not clear because increased, reduced, or unchanged kernel weights have been observed with an increased seeding rate (Donaldsonet al. 2001; Woodet al. 2003; Hiltbrunneret al. 2005). High N application rates increase the spike number per unit area and the grain number per spike, but decrease the thousand-kernel weight(Abadet al. 2004; Tianet al. 2011). Grain yield increases significantly in response to a properly increased seeding rate and N application rate (Fanget al. 2010; Daiet al. 2013;Hawkesford 2014). For example, with an optimized date of N topdressing,grain number per spike and grain weight can be increased significantly(Panet al. 2001; Zhuet al.2002). The ear-bearing tiller percentage can be enhanced by increasing the N topdressing ratio (Luet al. 2007) but is decreased by excessive inputs of N and early application of N fertilizer (Luet al. 2016). In addition, delayed sowing of winter wheat mainly shortens the duration of the vegetative stage (Sunet al.2007), thus reducing tiller development, spike number per unit area, and kernel weight (Thillet al. 1978),and ultimately decreasing the grain yield (Ortiz-Monasterioet al. 1994).

?

In our study, there were significant differences in yield components between the integrated treatments.The higher tiller number at jointing stage and higher ear-bearing tiller percentage were obtained with the T4 treatment, which ultimately resulted in the highest spike number per unit area. The single-spike dry weight and single-stem biological yield at anthesis stage were also affected by the integrated agronomic practices. Both of these traits were positively correlated with grain number per spike, in complete agreement with previous studies by Gonzálezet al. (2011b) and Zhanget al. (2012). The lowest grain number per spike was obtained with T4 and the highest grain number was obtained with T1. This was mainly due to T4 and T1 having the lowest and highest single-spike dry weight and single-stem biological yield at anthesis stage, respectively. The grain filling rate was also affected significantly by agronomic practice, but no significant differences in active filling duration were observed among T1, T3, and T4. In agreement with previous studies of Wiegand and Cuellar (1981) and Wanget al. (2012), our data clearly demonstrated a positive correlation between thousand-kernel weight and the average grain filling rate.The heaviest grain weight was obtained with T2, and the lightest grain weight was obtained with T1. This was mainly due to T1 and T2 having the highest and lowest grain filling rates, respectively.

The highest grain number per spike was observed with the T1 treatment. Despite this, grain yield with T1 was the lowest because wheat grown under this treatment also had the lowest spike number per unit area and thousand-kernel weight. The higher spike number per unit area in the T4 treatment compensated for reductions in both grain number per spike and thousand-kernel weight compared with T2,thus producing a higher grain yield than T2. However, the higher spike number per unit area with T4 compared with T3 did not compensate for the marked decrease in grain number per spike, which led to lower grain yield than that with T3.

NUE is a complex trait comprising two key major components, UPE and UTE. The UPE was determined based on both AGN and available N. UTE is influenced positively by NHI but negatively by GNC (Foulkeset al.2009). Properly increasing the seeding rate (Daiet al.2013, 2014), increasing the N application (Pasket al. 2012;Zhanget al. 2016), postponing the date and increasing the ratio of N topdressing (Palta and Fillery 1993; Bly and Woodward 2003; Menget al. 2013) are widely known to increase AGN. However, late sowing may weaken the N uptake (Widdowsonet al. 1987) and resulted in lower AGN and UPE (Daiet al. 2017). NHI tended to decrease with an increasing seeding rate (Daiet al. 2013) or N application rate(Ehdaie and Waines 2001; Pasket al. 2012). Additionally,NHI was higher (Kouret al. 2012) or unchanged (Daiet al.2017) with late sowing compared to that with early and normal sowing. Previous studies have shown that increased seeding rate has differential effects on GNC (Geletaet al.2002; Ozturket al. 2006; Daiet al. 2013), delayed sowing also cause different variance in GNC (McLeodet al. 1992;Ozturket al. 2006; Alignanet al. 2009; Gaoet al. 2012) and increased N fertilization rate can increase GNC (Barracloughet al. 2010).

In present study, T1 and T3 had the same quantity of N application rate and similar available N. However, the lowest AGN was observed with T1 resulting from the lower N topdressing ratio, earlier topdressing date, and lower seeding rate. Despite of the lowest available N, wheat grown under the T2 treatment obtained a higher AGN compared with T1 due to the optimized N topdressing ratio and date,and the higher seeding rate. While the lower AGN with T4 treatments was mainly due to the lower N application rates compared to T3. T1 had a relatively high available N and produced the lowest AGN, as a result of which observed the lowest UPE among all treatments, while T2 resulted in the highest UPE among all treatments due to the fact that this treatment had the lowest available N. Despite having a higher AGN, wheat grown under the T3 treatment produced a lower UPE compared with T4 due to the higher available N resulting from both a higher N fertilization rate and a higher level of mineralized N (NH4+-N and NO3–-N) in the soil.

N accumulation in grains at maturity stage exhibited the same ranking among the treatments as AGN. Although wheat grown under T3 had both higher N accumulation in grains and AGN compared with T4, we observed a relatively lower percent increase in N accumulation in grains compared with AGN, which led to the reduced NHI observed for T3.No significant difference in GNC was observed between T1 and T2. GNC was the highest for T3 and the lowest for T1 over the three years, and GNC averaged across the three years for T4 was 4.04% lower than that for T3. The higher UTE for T4 can be attributed to its higher NHI and lower GNC compared with T3.

T4 showed a higher NUE than T3 and T1 and also had both higher UPE and UTE. However, a higher NUE was observed for T2 than for T4, which resulted from a higher (or equal) UPE and a higher UTE. This indicates that further improvements in NUE are possible in the T4 management strategy.

Although the grain yield of T4 was not the highest, T4 showed the highest net profit among the four treatments.The higher net profit of T4 compared with T3 originated from the reduction of inputs, such as fertilizer (urea, calcium superphosphate and potassium chloride), irrigation, and labor.

5. Conclusion

By increasing the seeding rate, delaying the sowing date and optimizing nutrient management, a significantly higher grain yield was achieved with the T4 treatment compared with T1 and T2, and grain yield was 95.85% of that with T3 across all three years. The NUE of T4 treatment was 95.36% of the NUE for treatment T2, which had the highest NUE. Compared with T3 and T1, T4 lead to increases in both UPE and UTE and resulted in a higher NUE. The increased UPE with T4 can be mostly attributed to the high AGN and relatively low available N, and the increased UTE in T4 was mainly due to the fact that this treatment also resulted in the highest NHI and a relatively low GNC. Taking high grain yield, high NUE, and high economic benefits into consideration, T4 can be considered a recommendable management strategy in the target region, although further improvements in NUE are required.

Acknowledgements

This work was supported by the National Basic Research Program of China (2015CB150404), the Special Fund for Agro-scientific Research in the Public Interest, China(201203096), and the Project of Shandong Province Higher Educational Science and Technology Program, China(J15LF07).

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